The present invention relates to a device for detecting the properties such as bending resistance or stiffness of sheets of paper or the like, that are used as conditions or criteria on the basis of which it is determined whether or not bank notes or bills are genuine or they are impaired or deteriorated.
Recently, sheets of paper such as bank notes are counted, discriminated and deposited or withdrawn by various machines. Deteriorated or flaccid bank notes are not suitable for handling by such machines and must be recovered or collected during their circulation.
Some of the devices for detecting the degree of deterioration of bank notes are disclosed in, for instance Japanese Patent (Unexamined) Publication Nos. 61-168,084, 61-168,085 and 61-169,983.
The above-mentioned devices have means to produce sound by striking a sheet of paper with an elastic body, by rubbing the surface of a sheet or by conveying a sheet through a bent conveying passage so as to cause the sheet to bend, and the sound thus produced is converted into an electrical signal, which represents the degree of deterioration of sheets.
However, in the known devices, it is very difficult to pick up sound thus produced. As a result, a space in which sound is picked up must be made a completely soundproof chamber, and mechanical noises of the device must be suppressed as much as possible so that there arises the problem that the devices become very expensive. Furthermore, even when the above-mentioned problems are solved, the erroneous detection of other sounds tends to occur frequently so that there arises the problem of erroneous detection of the properties of sheets.